Road surface friction coefficient estimating device and road surface friction coefficient estimating method

Abstract

A road surface friction coefficient estimating device includes a lateral force detecting section for detecting the lateral force of a wheel during traveling, a slip angle detecting section for detecting the slip angle of the wheel during traveling, and a road surface μ calculating section for estimating the relationship between the detected lateral force and the detected slip angle on the basis of the ratio between the detected lateral force and the detected slip angle, the correlation between the lateral force and the slip angle in the case of the reference road surface, and at least either the detected lateral force or the detected slip angle.

The invention claimed is: 
     
       1. A road surface friction coefficient estimating device comprising:
 a lateral force detecting section that detects a lateral force of a wheel; 
 a slip angle detecting section that detects a slip angle of the wheel; and 
 a road surface friction coefficient estimating section that:
 stores information about a characteristic curve in a coordinate plane, wherein the coordinate plane has a coordinate axis representing the lateral force and a coordinate axis representing the slip angle, and wherein the characteristic curve represents a relationship between the lateral force and the slip angle under a condition of a reference road surface friction coefficient; 
 finds with reference to the stored information a point in the coordinate plane at which a straight line intersects with the characteristic curve, wherein the straight line passes through an origin point of the coordinate plane and a detected point, and wherein the detected point corresponds to a detected value of the lateral force obtained by the lateral force detecting section and a detected value of the slip angle obtained by the slip angle detecting section; and 
 calculates an estimated value of a road surface friction coefficient, based on a reference value and the detected value of at least one of the lateral force and the slip angle and the reference road surface friction coefficient, wherein the reference value corresponds to the found point. 
 
 
     
     
       2. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the road surface friction coefficient estimating section:
 calculates a gradient of the straight line in the coordinate plane by dividing the detected value of the lateral force by the detected value of the slip angle; and 
 finds the found point based on the gradient. 
 
     
     
       3. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the coordinate plane has a horizontal axis representing the slip angle and a vertical axis representing the lateral force. 
     
     
       4. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the origin point of the coordinate plane is a point at which the lateral force is equal to zero. 
     
     
       5. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the road surface friction coefficient estimating section:
 calculates a ratio to reference based on the reference value and the detected value of at least one of the lateral force and the slip angle; and 
 calculates the estimated value of the road surface friction coefficient based on the ratio to reference and the reference road surface friction coefficient. 
 
     
     
       6. The road surface friction coefficient estimating device as claimed in  claim 5 , wherein the road surface friction coefficient estimating section calculates the ratio to reference by dividing the detected value of the at least one of the lateral force and the slip angle by the reference value of the at least one of the lateral force and the slip angle. 
     
     
       7. The road surface friction coefficient estimating device as claimed in  claim 5 , wherein the road surface friction coefficient estimating section:
 calculates as a first distance a distance in the coordinate plane between the detected point and a point at which the lateral force is equal to zero; 
 calculates as a second distance a distance in the coordinate plane between the found point and a point at which the lateral force is equal to zero; and 
 calculates the ratio to reference based on the first and second distances. 
 
     
     
       8. The road surface friction coefficient estimating device as claimed in  claim 7 , wherein the road surface friction coefficient estimating section calculates the ratio to reference by dividing the first distance by the second distance. 
     
     
       9. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the road surface friction coefficient estimating section stores information about a second characteristic curve in a second coordinate plane,
 wherein the second coordinate plane has a horizontal axis representing a ratio of the lateral force to the slip angle and a vertical axis representing the lateral force, and 
 wherein the second characteristic curve represents a relationship between the lateral force and the slip angle under a condition of the reference road surface friction coefficient. 
 
     
     
       10. The road surface friction coefficient estimating device as claimed in  claim 9 , wherein the road surface friction coefficient estimating section:
 calculates as a ratio to reference a ratio of the detected value of the lateral force to the reference value of the lateral force with reference to the information about the second characteristic curve; and 
 calculates the estimated value of road surface friction coefficient based on the ratio to reference and the reference road surface friction coefficient. 
 
     
     
       11. The road surface friction coefficient estimating device as claimed in  claim 10 , wherein the road surface friction coefficient estimating section calculates the ratio to reference by dividing the detected value of the lateral force by the reference value of the lateral force. 
     
     
       12. The road surface friction coefficient estimating device as claimed in  claim 1 , wherein the road surface friction coefficient estimating section stores information about a second characteristic curve in a second coordinate plane,
 wherein the second coordinate plane has a horizontal axis representing a ratio of the lateral force to the slip angle and a vertical axis representing the slip angle, and 
 wherein the second characteristic curve represents a relationship between the lateral force and the slip angle under a condition of the reference road surface friction coefficient. 
 
     
     
       13. The road surface friction coefficient estimating device as claimed in  claim 12 , wherein the road surface friction coefficient estimating section:
 calculates as a ratio to reference a ratio of the detected value of the slip angle to the reference value of the slip angle with reference to information about the second characteristic curve; and 
 calculates the estimated value of road surface friction coefficient based on the ratio to reference and the reference road surface friction coefficient. 
 
     
     
       14. The road surface friction coefficient estimating device as claimed in  claim 13 , wherein the road surface friction coefficient estimating section calculates the ratio to reference by dividing the detected value of the slip angle by the reference value of the slip angle. 
     
     
       15. The road surface friction coefficient estimating device as claimed in  claim 13 , wherein the road surface friction coefficient estimating section calculates the estimated value of the road surface friction coefficient by multiplying the reference road surface friction coefficient by the ratio to reference. 
     
     
       16. A road surface friction coefficient estimating method comprising performing:
 an operation of detecting, by a controller, a lateral force of a wheel; 
 an operation of detecting, by the controller, a slip angle of the wheel; and 
 an operation of:
 storing, by the controller, information about a characteristic curve in a coordinate plane, wherein the coordinate plane has a coordinate axis representing the lateral force and a coordinate axis representing the slip angle, and wherein the characteristic curve represents a relationship between the lateral force and the slip angle under a condition of a reference road surface friction coefficient; 
 finding, by the controller, with reference to the stored information, a point in the coordinate plane at which a straight line intersects with the characteristic curve, wherein the straight line passes through an origin point of the coordinate plane and a detected point, and wherein the detected point corresponds to a detected value of the lateral force obtained by the lateral force detecting and a detected value of the slip angle obtained by the slip angle detecting; and 
 calculating, by the controller, an estimated value of road surface friction coefficient, based on a reference value and the detected value of at least one of the lateral force and the slip angle, and the reference road surface friction coefficient, wherein the reference value corresponds to the found point.